Reverse Linked List

1package pkg5leetcode.blind75;2 3/*4 * Reverse Linked List | LC 2065 * APPROACH: Iterative three-pointer reversal.6 * COMPLEXITY: Time O(n), Space O(1)7 */8public class blind75_LC206ReverseLinkedList {9    /** Same shape as pkg5leetcode/common/ListNode.java (nested for single-file runs). */10 11    static class ListNode {12        int val;13        ListNode next;14        ListNode(int val) { this.val = val; }15    }16 17    static ListNode reverseList(ListNode head) {18        ListNode prev = null, cur = head;19        while (cur != null) {20            ListNode nxt = cur.next;21            cur.next = prev;22            prev = cur;23            cur = nxt;24        }25        return prev;26    }27 28    static int[] toArray(ListNode head) {29        java.util.List<Integer> list = new java.util.ArrayList<>();30        while (head != null) { list.add(head.val); head = head.next; }31        return list.stream().mapToInt(Integer::intValue).toArray();32    }33 34    public static void main(String[] args) {35        ListNode a = new ListNode(1); a.next = new ListNode(2); a.next.next = new ListNode(3);36        check(java.util.Arrays.equals(toArray(reverseList(a)), new int[]{3, 2, 1}), "case1");37        ListNode b = new ListNode(1);38        check(java.util.Arrays.equals(toArray(reverseList(b)), new int[]{1}), "case2");39        System.out.println("all tests passed");40    }41 42    static void check(boolean cond, String name) {43        if (!cond) throw new AssertionError("FAILED: " + name);44        System.out.println("  PASS " + name);45    }46}